Refractory metals and their silicides are often used in semiconductor manufacture. One silicide that is of particular interest in the formation of MOS and CMOS integrated circuits is tungsten silicide. Tungsten silicide, generally in the form of (WSi.sub.2), can be used in the formation of integrated circuits as an intermediate, barrier, or conducting film. Tungsten silicide can also be used with polysilicon as a polycide-gate in transistor formation. Among the useful properties of tungsten silicide are its low bulk resistance and low stress.
In the formation of a tungsten silicide film, the film quality and step coverage provided by the deposited film greatly influences the completed semiconductor devices. This is because poor step coverage and an irregular surface topography generated by multi-layer processes causes serious difficulties for subsequent processing steps such as lithography, deposition, and etching. Poor step coverage and high stress may also degrade device reliability by causing stress induced cracking. Moreover, impurities in the deposited film may degrade device reliability and cause metal migration.
Chemical vapor deposition (CVD) is typically utilized in the deposition of tungsten silicide. In general (CVD) is favored because the deposited film has lower impurities and better step coverage than a sputtered film. In such a (CVD) system the reaction chemistry is typically based on the reaction of a silicon source gas, such as silane (SiH.sub.4), with a reactant gas, such as tungsten hexaflouride (WF.sub.6). One advantage of a (CVD) deposition process of tungsten silicide using this reaction chemistry is that because of the high reactivity of silane (SiH.sub.4) and tungsten hexaflouride (WF.sub.6), relatively low process temperatures can be utilized. These deposition temperatures are typically in the range of 250.degree.-400.degree. C. A disadvantage of a (CVD) deposition process of tungsten silicide using this reaction chemistry is the very high levels of fluorine impurities in the deposited film. In addition, step coverage may be poor.
In order to provide a tungsten silicide film having a lower impurity content and a better step coverage, a (CVD) reaction chemistry using higher order silanes such as dislane dichlorosilane and trislane have been proposed. U.S. Pat. Nos. 4,684,542 to Jasinski et al. and U.S. Pat. No. 4,966,869 to Hillman et al. disclose such processes.
Such a process is also disclosed in the technical report entitled, "Properties of WSi.sub.x using dichlorosilane in a single-wafer system" reported in the J. Vac. Sci. Technol. B 6(6), Nov/Dec 1988. This process was carried out in a (CVD) cold-wall reactor using a reaction of tungsten hexafluoride (WF.sub.6) and dichlorosilane (SiH.sub.2 Cl.sub.2) gases.
In this prior art process plasma enhancement of the process gases using radio frequency-induced glow discharge, was required to obtain the thermal deposition. The resultant film was characterized by a much lower fluorine concentration than films deposited with a conventional WF.sub.6 /SiH.sub.4 chemistry. In addition, films deposited with a dichlorosilane source gas are characterized by excellent step coverage.
Although this process allowed the tungsten silicide film to be deposited at relatively low process temperature (450.degree.-650.degree. C.), the requirement of plasma enhancement for thermal deposition tended to complicate the procedure. Such a complicated procedure may not be entirely suitable for large scale semiconductor manufacture. In addition plasma can cause degradation of devices due to radiation induced damage.
There is then a need in the art for a process suitable for large scale semiconductor manufacture for forming high quality tungsten silicide films at relatively low process temperatures. Accordingly, it is an object of the present invention to provide a process for depositing tungsten silicide at a relatively low process temperature. It is another object of the present invention to provide a process for depositing a tungsten silicide film using dichlorosilane as a silicon source gas such that a high quality film having low impurities, low stress, and good step coverage is provided. It is yet another object of the present invention to provide a process for depositing a high quality tungsten silicide film that is adaptable to large scale semiconductor manufacture.